This invention relates generally to highlight color imaging and more particularly to two-pass highlight color imaging.
In the practice of conventional xerography, it is the general procedure to form electrostatic latent images on a xerographic surface by first uniformly charging a charge retentive surface such as a photoreceptor. Only the imaging area of the photoreceptor is uniformly charged. The image area does not extend across the entire width of the photoreceptor. Accordingly, the edges of the photoreceptor are not charged. The charged area is selectively dissipated in accordance with a pattern of activating radiation corresponding to original images. The selective dissipation of the charge leaves a latent charge pattern on the imaging surface corresponding to the areas not exposed by radiation.
This charge pattern is made visible by developing it with toner by passing the photoreceptor past a single developer housing. The toner is generally a colored powder which adheres to the charge pattern by electrostatic attraction. The developed image is then fixed to the imaging surface or is transferred to a receiving substrate such as plain paper to which it is fixed by suitable fusing techniques.
In tri-level, highlight color imaging, unlike conventional xerography, the image area contains three voltage levels which correspond to two image areas and to a background voltage area. One of the image areas corresponds to non-discharged (i.e. charged areas) of the photoreceptor while the other image areas correspond to discharged areas of the photoreceptor. The charged areas are developed using Charged Area Development (CAD) while the discharged areas are developed using Discharged Area Development (DAD).
The concept of tri-level, highlight color xerography is described in U.S. Pat. No. 4,078,929 issued in the name of Gundlach. The patent to Gundlach teaches the use of tri-level xerography as a means to achieve single-pass highlight color imaging. As disclosed therein the charge pattern is developed with toner particles of first and second colors. The toner particles of one of the colors are positively charged and the toner particles of the other color are negatively charged. In one embodiment, the toner particles are supplied by a developer which comprises a mixture of triboelectrically relatively positive and relatively negative carrier beads. The carrier beads support, respectively, the relatively negative and relatively positive toner particles. Such a developer is generally supplied to the charge pattern by cascading it across the imaging surface supporting the charge pattern. In another embodiment, the toner particles are presented to the charge pattern by a pair of magnetic brushes. Each brush supplies a toner of one color and one charge. In yet another embodiment, the development systems are biased to about the background voltage. Such biasing results in a developed image of improved color sharpness.
In highlight color xerography as taught by Gundlach, the xerographic contrast on the charge retentive surface or photoreceptor is divided three, rather than two, ways as is the case in conventional xerography. The photoreceptor is charged, typically to 900 v. It is exposed imagewise, such that one image corresponding to charged image areas (which are subsequently developed by charged-area development, i.e. CAD) stays at the full photoreceptor potential (V.sub.cad or V.sub.ddp, shown in FIG. 1a). The other image is exposed to discharge the photoreceptor to its residual potential, i.e. V.sub.dad or V.sub.c (typically 100 v) which corresponds to discharged area images that are subsequently developed by discharged-area development (DAD) and the background areas exposed such as to reduce the photoreceptor potential to halfway between the V.sub.cad and V.sub.dad potentials, (typically 500 v) and is referred to as V.sub.white or V.sub.w. The CAD developer is typically biased about 100 v (V.sub.bb, shown in FIG. 1b) closer to V.sub.cad than V.sub.white (about 600 v), and the DAD developer system is biased about 100 v (V.sub.cb, shown in FIG. 1b) closer to V.sub.dad than V.sub.white (about 400 v).
Another method of highlight color imaging is the two-pass technique. In the two-pass system the charge retentive surface is moved through the processing stations twice. In two-pass highlight color imaging it is necessary to render development inoperative in the inter-document gap in order to avoid certain development problems. Single pass schemes like the tri-level concept of Gundlach discussed above avoid the requirement of rendering the development inoperative in the inter-document gap by keeping both development systems engaged or operative and using suitably biased, opposite polarity developer to develop both images sequentially within the same frame. The trade-off using tri-level imaging in lieu of two-pass imaging is the necessity of imaging three light levels within one frame (i.e. black, white and color) thereby cutting the voltage latitude in half or more. This necessitates using a high gamma development system like conductive mag brush (CMB). In two-pass highlight color imaging, the full contrast voltage is substantially available for each of the two images.
Present two-pass system concepts have either had to cam development housings in and out within the inter-document gap or else keep both housings in the development zone but turn the flow of developer off and the other one on in the gap. These approaches either produce mechanical problems that limit the process speed or else necessitate very high tolerances which may become formidable especially with an insulative magnetic brush development system where the charge retentive surface is wrapped partly around the developer rolls.